CODEWITHSUNDEEP
matlabmatrix
popist
popist

Reputation: 83

Fill a zeros matrix with specific numbers of 1

I'm facing a problem. I have a zeros matrix 600x600. I need to fill this matrix with 1080 1s randomly. Any suggestions?

Upvotes: 1

Views: 321

Answers (3)

patrik
patrik

Reputation: 4558

This is one way to do it,

N = 1080; % Number of ones
M = zeros(600); % Create your matrix
a = rand(600^2,1); % generate a vector of randoms with the same length as the matrix
[~,asort] = sort(a); % Sorting will do uniform scrambling since uniform distribution is used
M(asort(1:N)) = 1; % Replace first N numbers with ones.

Upvotes: 2

Adriaan
Adriaan

Reputation: 18187

A = sparse(600,600);               %// set up your matrix
N=1080;                            %// number of desired ones
randindex = randi(600^2,N,1);      %// get random locations for the ones
while numel(unique(randindex)) ~= numel(randindex)
    randindex = randi(600^2,N,1);  %// get new random locations for the ones
end 
A(randindex) = 1;                  %// set the random locations to 1

This utilises randi to generate 1080 numbers randomly between 1 and 600^2, i.e. all possible locations in your vectors. The while loop is there in case it happens that one of the locations occurs twice, thus ending up with less than 1080 1.

The reason you can use a single index in this case for a matrix is because of linear indexing.

The big performance difference with respect to the other answers is that this initialises a sparse matrix, since 1080/600^2 = 0.3% is very sparse and will thus be faster. (Thanks to @Dev-iL)

Upvotes: 3

High Performance Mark
High Performance Mark

Reputation: 78364

Or, use the intrinsic routine randperm thusly:

A = zeros(600);
A(randperm(600^2,1080)) = 1;

Upvotes: 6

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